In recent years and continuing, orthogonal frequency division multiplexing (OFDM) has been attracting attention as a transmission scheme of wireless communications. OFDM is robust against multipath propagation and efficient in use of frequency. In an OFDM system, multiple sub-carriers orthogonal to each other are provided in a transmission band, and data values are allocated to the amplitude and the phase of each sub-carrier to perform digital modulation. Because multiple sub-carriers are transmitted in parallel, the frequency band allocated to each sub-carrier becomes narrow. The amount of signals per symbol time is reduced, and therefore, the modulation rate is slow. Meanwhile, because of the low modulation rate, OFDM is insusceptible to multipath interference.
In an OFDM receiver, various types of synchronization processes are performed on the demodulation circuit. As for a clock signal that has an effect on the entire process of demodulation, a difference in the oscillation frequencies between a transmitter's clock oscillator and a receiver's clock oscillator causes a synchronization offset, and the signal receiving performance is degraded. In order to synchronize the receiver's clock frequency with the transmitter's clock frequency, the receiver's clock frequency is generally corrected.
Besides, the carrier frequency of a sub-carrier may vary due to frequency offset and phase offset of a local oscillator. In this case, orthogonality between sub-carriers of the OFDM signals are lost, and inter-carrier interference will occur. Accordingly, correction for the carrier frequencies of sub-carriers is also important for the signal receiving quality.
It has been proposed to make use of pilot signals, which are known signals scattered in an OFDM frame, to correct a carrier frequency or a clock frequency. See, for example, Japanese Laid-open Patent Publication No. 2002-290373. With this technique, the characteristics of the amplitude and the phase of a pilot signal are detected, and inverse fast Fourier transform (IFFT) is performed on the detected characteristics. The signal component acquired through the IFFT is used to correct both the carrier frequency and the clock frequency.
To be more precise, a peak position of the desired wave component is detected from the IFFT output, and a carrier phase error (which corresponds to a carrier frequency error) is detected from the amplitude and the phase at the peak position. In addition, based upon a delay time offset at the peak position of the desired wave, symbol synchronization phase error and clock phase error, which phase errors relate to a clock frequency error, are detected. Then, appropriate corrections are made to the carrier phase and the clock phase, corresponding to the carrier phase error and the clock phase error, respectively.
By using a delay profile acquired from the IFFT computation, desired waves and interfering waves can be separated according to a delay time. Hence, carrier correction and clock correction based upon a delay profile are insusceptible to multipath interference.
However, in a multipath environment where two or more desired waves exist at different delay times and where the phase excursions (or carrier frequencies) of the desired waves are different from each other, a problem will occur. When performing carrier correction based upon the phase excursion of one of the desired waves, an offset will occur from the phase excursions (or the carrier frequencies) of the other desired waves, and the signal receiving quality is degraded.
In addition, when detecting a clock phase error using the delay time offset of the desired wave component contained in the IFFT output, the resolution performance and the maximum delay time analyzable from the IFFT output values depend on the number of points of IFFT computation and the input interval of frequencies. If the number of IFFT points is insufficient, the resolution of clock error detection is degraded.
Hence, if there are several desired waves at different delay times, and if the carrier frequencies and the clock frequencies are different among the desired waves, synchronization correction process may not converge at the optimum position.
It is desired to provide a technique of appropriate correction for synchronization offsets of carrier frequencies and clock frequencies of desired waves even if there are multiple desired waves at different delay times and different carrier frequencies.